ISTANBUL  - Syria said Friday it was ready to swap prisoners with rebels and to help improve humanitarian aid as the divided Syrian opposition debates whether to join international peace talks next week.

Foreign Minister Walid Muallem offered the concessions at key preparatory talks in Moscow for the so-called Geneva II peace conference aimed at finding a way out of almost three years of brutal conflict.

His announcement could mark another diplomatic success for Russia after the Kremlin managed to convince Damascus last year to renounce its chemical weapons to avert US air strikes.

It came as the deeply-divided Syrian opposition -- under intense pressure from Western and Arab allies -- was to meet in Istanbul to decide whether to join the talks opening in Switzerland on January 22.

The long-delayed Geneva II is aimed at finding a way to install a transitional government to help chart an end to the civil war, which has cost 130,000 lives since March 2011 and sent millions of people fleeing. Muallem said Syria would "make every effort to ensure this event is a success and meets the aspirations of the Syrian people and the direct orders of President Bashar al-Assad".

He said Damascus was prepared to exchange prisoners with rebel forces but declined to specify how many or when the operation might take place.

Muallem also offered to take a "series of humanitarian steps" to lead to the speedy delivery of aid to those in need, Russian Foreign Minister Sergei Lavrov said. But Muallem failed to address a joint call this week by Lavrov and US Secretary of State John Kerry for a ceasefire in parts of Syria that could begin in the devastated northern city of Aleppo.

Kerry also Thursday appealed to the umbrella National Coalition opposition not to boycott the Geneva talks because they represented "the best opportunity for the opposition to achieve the goals of the Syrian people and the revolution".

"The Syrian people need to be able to determine the future of their country, their voice must be heard," he told reporters.

But parts of the opposition are wary of being drawn into a process they fear could result in Assad clinging to power and had set his departure as a condition for joining the talks.

'Between a rock and a hard place'

UN refugee chief Antonio Guterres said Friday it was vital to find a political solution to the conflict as he appealed for the world to ease the massive burden on countries which have taken in millions of Syrian refugees.

The United Nations earlier this week launched a massive $6.5 billion appeal for humanitarian aid.

"For me it is unacceptable to see Syrian refugees drowning, dying in the Mediterranean or pushed back at some borders," Guterres said at a meeting of refugee-hosting countries in Turkey.

Friday's opposition meeting in Istanbul -- which comes more than a week after the coalition failed to agree a united stance -- is expected to be fraught.

"The debate will be long and difficult because the coalition leadership is caught between a rock and a hard place," said one Western diplomat.

"It is likely in the end that the coalition will send a delegation to Geneva but at what cost to its future."

A key bloc, the Syrian National Council, has already threatened to pull out of the coalition if it votes in favour of attending Geneva II.

Complicating the situation are the fierce battles between mainstream rebels and Al-Qaeda linked jihadists that monitors say have killed over 1,000 people in two weeks. A senior Damascus official said this week that Western intelligence officials visited Syria to discuss joint security cooperation against the radical Islamist fighters, but this was denied by US officials.

The Syrian regime had warned Monday against setting preconditions for the talks that kick off in the Swiss lakeside town of Montreux.

"Any person who seeks preconditions or mistakes their dreams for reality is leading to the failure of the Geneva conference before it even starts," Syrian state media quoted a foreign ministry source as saying.

But Kerry sought to allay opposition fears that the talks would somehow legitimise Assad's regime and leave him in power, saying any names put forward for the transitional leadership must be agreed by both sides.

"This means that any figure that is deemed unacceptable by either side, whether president Assad or a member of the opposition, cannot be a part of the future."

British media reported this week that the United States and Britain had even threatened to cut support to the opposition if it failed to send a delegation to Switzerland.

"They are making it very clear that they will not continue to support us the way they are doing now and that we will lose credibility with the international community if we do not go," an unnamed senior coalition official was quoted as saying.